1. Field of the Invention
The present invention relates generally to a method and apparatus for thermal insulation of wet shielded metal arc welds.
2. Description of the Prior Art
Underwater welding processes can be classified as dry or wet welding techniques based upon whether or not the welds are physically isolated from the surrounding water environment. Welds that are physically isolated from the surrounding water are classified as "dry welds" while welds which are in contact with the surrounding water are classified as "wet welds."
Dry welding techniques in current use and compatible with shielded metal arc (SMA) (as will be defined in a following section of this specification) processes include one atmosphere welding, habitat welding, dry chamber welding and dry spot welding. One atmosphere welding is generally performed in a pressure vessel maintained at approximately one atmosphere absolute. Habitat welding is performed in an open-bottom chamber having a complete atmosphere conditioning system which removes welding and diver exhaled respiratory gases and provides breathing air, thus allowing a diver to weld without wearing diving equipment. Dry chamber welding is performed in a pressurized open-bottom enclosure by a diver wearing diving equipment from which welding and exhaled respiratory gases must also be vented. In dry spot welding processes, water is displaced from the immediate vicinity of the weld area by a transparent gas-filled box or by a flow of shielding gas surrounded by a concentric ring of water jets. Dry spot welding apparatus are moved along the weld line during the welding process.
In a wet shielded metal arc welding (SMAW) process, an arc is struck in the water between an electrode and the surface being welded and the weld is formed as the welder moves the electrode along a weld line. No active thermal treatment of wet shielded metal arc welds is feasible because the welds are in direct contact with the water environment.
Typically, wet shielded metal arc welding is faster and less expensive than the already-mentioned dry welding processes and, additionally, can be used to make welds on a greater variety of geometrically complex structures. However, the use of SMAW processes is currently limited to only noncritical or temporary joints in mild steel marine structures because of the inferior quality of wet SMA welds by comparison with dry SMA welds. Wet SMA welds are generally more brittle, more porous and more crack susceptible than dry SMA welds made under otherwise equivalent arc voltage, arc current, and arc travel speed welding conditions using the same workpiece, electrode and flux materials.
The poor quality of wet SMAW welds is primarily attributed to rapid weld quenching by the surrounding water environment. During a wet SMAW process, the SMA welding arc is protected from the surrounding water by a bubble of gas produced as the electrode flux decomposes. However, the weld puddle left behind by the arc is immediately exposed to the surrounding water after arc passage, so that by contrast with those of dry SMA welds or SMA welds made on the surface in air, wet SMA weld puddles are subjected to quenching.
Quenching compromises weld quality by causing the formation of martensite in the heat affected zone (HAZ) of the weld. The HAZ is defined as the section of the weld that has been heated to above the A1 transformation temperature (723.degree. C.) for mild steel but has not reached the melting point of the material being welded. If the rate of cooling of a steel from the transformation temperature to ambient temperature is sufficiently rapid, martensite is formed. Since the crystal structure of untempered martensite inhibits dislocation movement, untempered martensite is non-ductile and its presence in the HAZ causes the weld to be brittle, hard and lacking in toughness.
Arc electrolysis of the surrounding water produces atomic hydrogen and oxygen gases which diffuse into the weld pool. These gases can become trapped in the final weld because the outward diffusion rate throughout the rapidly solidifying metal is much slower than the initial gas diffusion rate into the molten weld pool. Entrapped hydrogen reduces weld ductility through what is known as the "cold cracking" mechanism. With time, hydrogen atoms in the weld migrate to interstitial voids where they recombine to form hydrogen gas. Pockets of hydrogen gas produce tensile stresses that can initiate trans-granular cracks. Since this phenomenon generally occurs after the weld has cooled to approximately 200.degree. C. the effect is referred to as "cold cracking". Oxygen gas trapped in the weld also produces pores which reduce weld strength and toughness.
Rapid quenching of the weld also traps non-gaseous contaminants, such as oxide slags, in the bulk of the weld metal. These contaminants are generally less dense than iron and float to the surface of the weld pool under gradual cooling conditions. Under rapid cooling conditions, they are entrapped in the bulk weld metal and further increase weld porosity.
In addition to microstructural and chemical effects. quenching produces steep thermal gradients with resultant high residual stresses, thereby increasing weld susceptibility to crack initiation upon exposure to environmental loading. Finally, weld quenching increases weld bead convexity which makes welds more susceptible to toe cracking.
In summary, wet SMAW welds are inherently brittle, porous and susceptible to cracking. All of these defects can be directly or indirectly attributed to weld quenching. Attempts to insulate underwater welds have included flux-shielding in a wet SMAW process and mechanical shielding of a GMAW torch with strips of moderately thermally resistant insulation.
Flux-shielding in a wet SMA process presents several technical difficulties among which are difficulty in enclosing the weld joint with the flux which permits use only with simple weld joint geometries and difficulty in keeping flux applied against a joint in an overhead or vertical orientation. Furthermore, as reported in Development of New Improved Techniques for Underwater Welding--MIT Sea Grant Program, Report No. MITSG 77-9, Chon-Liang Tsai et al., pages Abstract; 26-27; 96-116, April, 1977, Massachusetts Institute of Technology, welds made with the flux-shielding process were inferior to welds made in air, based on the presence of harder weld metal deposits and HAZ's in the wet SMA flux-shielded welds. Finally, the presence of the flux obscures the welder's view of the weld joint which is undesirable since visual observation of the weld joint is critical to a manual welder's control of the welding process. As part of this study, weld joint insulation with 1/8 inch insulator strips made of asbestos, a naturally occurring, hazardous and toxic material was also examined.
The Satoh et al. "Study on improvement of Locally Drying Underwater Welding Joint by Retart[d]ed Cooling Method", pages 47-54, 1982, Journal of the Japanese Welding Society, 51 method of attaching thermal insulation to a GMAW torch includes insulation strips extending from the torch parallel to the weld line to shield the weld from water both immediately before and after arc passage as well as insulation strips arranged perpendicular to the weld line and in contact with the parallel insulation strips, a shielding configuration which has the undesirable effect of blocking the welder's view of the weld joint and weld line.
Thus, there exists a need for a method for thermally insulating a wet SMA weld wherein thermal insulation made of non-toxic, safely handled material having sufficiently low thermal conductivity to adequately prevent too rapid cooling of the weld joint can be readily applied to the workpiece without interfering with the welder's view of the weld joint, welding arc, weld line or weld puddle.